1. Field of the Invention
This invention relates generally to an undesired signal canceller, and more particularly to a ghost signal canceller for use in a television receiver.
2. Description of the Prior Art
In the art, a television receiver, by way of example, receives not only a broadcasting wave directly from a transmitting antenna but also waves reflected from mountains, buildings and so on, so that the video picture appears to be reproduced on the television receiver with a double or triple image; that is, the so-called ghost image is reproduced.
One example of prior art ghost signal cancellers can best be explained by referring to FIG. 1 of the accompanying drawings. With this prior art ghost signal canceller, the output terminal of a video detector circuit 2, whose input terminal 1 is supplied with an IF(intermediate frequency) signal, is connected with a plurality of variable delay lines 3.sub.1, 3.sub.2, . . . 3.sub.n in parallel with one another. The output signals from the variable delay lines 3.sub.1, 3.sub.2, . . . 3.sub.n are fed through level controllers 4.sub.1, 4.sub.2, . . . 4.sub.n to an adder circuit 5 to be added, respectively. The output video signal from the video detector circuit 2 and the output signal from the adder circuit 5 are both supplied to a subtracter circuit 6 to thereby cancel ghost signals and to obtain only a desired signal at an output terminal 7 of from the substracter circuit 6. The above prior art ghost signal canceller proceeds on the assumption that ghost signals, which are low in level and delayed in phase as compared with a desired signal, are superimposed on the desired signal. However, where the ghost signal is produced by adding high frequency modulated signals (television wave signals) with one another at a receiving antenna, such a ghost signal generally is not merely low in level and delayed in phase as compared with the desired signal. Therefore, the latter ghost signals can not be cancelled or eliminated sufficiently by the prior art ghost signal canceller shown in FIG. 1.
FIGS. 2A to 2E show respectively different waveforms which result from different phase differences between a desired signal and a ghost signal. It is assumed that when a desired signal S.sub.h shown in FIG. 2A is transmitted, a ghost signal S.sub.g appears at a certain receiving point or time. When the desired signal S.sub.h and the ghost signal S.sub.g are received as a composite signal and envelope-detected, the envelopes of the composite signal are respectively shown in FIGS. 2B to 2E due to the phase relation of the carrier signals of the desired and ghost signals S.sub.h and S.sub.g. That is, when the phase difference between the desired and ghost-carrier signals is 0.degree., the desired and ghost signals S.sub.h and S.sub.g are of the same polarity as shown in FIG. 2B, but when the phase difference is 180.degree., the desired and ghost signals S.sub.h and S.sub.g are of opposite polarity as shown in FIG. 2D. If the phase difference is 0.degree. or 180.degree., the ghost signal S.sub.g can be eliminated by the prior art ghost signal canceller shown in FIG. 1.
However, when the phase difference is 90.degree. or 270.degree.; the ghost signal S.sub.g appears as the waveform as shown in FIG. 2C or 2E which appears as a differentiated version of the ghost signal S.sub.g when the phase difference is 0.degree. or 180.degree. shown in FIG. 2B and 2D. In such a case, that is, when the phase difference is 90.degree. or 270.degree., the ghost signal S.sub.g can not be eliminated by the ghost signal canceller shown in FIG. 1.
FIG. 3 shows another prior art ghost signal canceller which differs from that shown in FIG. 1 in that the IF signal is treated in advance of the video detector to carry out ghost signal cancelling before the IF signal is supplied to the video detector 2 (not shown in FIG. 3). The remaining circuit construction of FIG. 3 is substantially the same as that of FIG. 1.
If the signal is treated in the form of an IF signal (FIG. 3) the ghost signal can be eliminated even when the phase difference between the desired signal and ghost signal is other than 0.degree. or 180.degree.. However, in this case the variable delay lines 3.sub.1, 3.sub.2, . . . 3.sub.n are of complex design and are large in size. A solid delay line whose delay time is fixed can not be used as the variable delay lines, but in general a plurality of phase shifters in cascade connection can be used effectively. An example of such variable delay lines is shown in FIG. 4 in which one terminal of a circuit 9a consisting of an inductance and a capacitor, which serves as a phase shifter, is used as an input terminal 10 and its other terminal is used as an output terminal 11a. Another similar circuit 9b, also consisting of a coil and a capacitor, is connected at its input terminal to the output terminal 11a of circuit 9a and its output terminal 11b is connected with the input terminal of a further circuit 9c, also consisting of a coil and a capacitor. Other similar circuits 9d, . . . as shown, are connected in cascade and output terminals 11c, 11d, . . . are led out therefrom, respectively. In one embodiment, these output terminals 11a, 11b, 11c, . . . are used as the fixed contacts of a slide switch 12 from whose movable contact 13 there is connected an output terminal 14. With such variable delay line circuit construction, the effective number of circuits connected between input and output terminals 10 and 14 is determined by the slide switch 12, and the delay time between the input and output terminals can be changed in a step-wise manner.
If the IF ghost signal is delayed by a time of .tau..sub.o with respect to the desired IF signal and the carrier of the ghost signal has the period T as shown in FIG. 5, the number of circuits 9a, 9b, . . . , (FIG. 4) required for eliminating the ghost signal is in proportion to .tau..sub.o/T since circuits 9a, 9b . . . serve as respective phase shifters. In fact, since the frequency of the carrier for the IF signal is high, for example, 58.75 MHz, so that its period T is very short, .tau..sub.o/T becomes large. As a result, the number of phase shifters forming the variable delay lines is increased substantially, resulting in an apparatus of great size. Further, the design of the variable delay lines becomes difficult in view of temperature compensation due to their large size and their accuracy becomes low. For this reason, the prior art ghost signal canceller shown in FIG. 3 does not operate completely satisfactorily to cancel ghost signals.